Several prior art check valves, such as those shown in U.S. Pat. Nos. 5,020,562 to Richmond et al., 3,831,629 to Mackal et al., 3,570,484 to Steer et al., 3,429,338 to Mackal et al., and 3,192,949 to De See, normally prevent flow through the valve and allow flow when actuated by a cannula. Although these valves have pistons that move within the fluid passageway, sealing does not occur at a sealing ring mounted on the periphery of the piston adjacent the passageway wall, but rather at the end of the piston or on a shoulder of the piston (not at the periphery of the shoulder). Thus, these valves do not need channels to permit flow past the sealing ring when the valve is opened.
U.S. Pat. No. 4,683,916 to Raines shows a normally closed valve wherein sealing is accomplished by a disk, which is deformed, when a cannula is inserted into the valve, in order to permit flow through the valve.
U.S. Pat. No. 4,143,853 to Abramson also shows a normally closed valve. A disk having a central axially extending slit normally prevents flow through the valve, and when actuated by a cannula, permits flow through the opened slit.